1. HEC-HMS Simulation Adding a detention pond at the outlet
Example 4
HEC-HMS Simulation
Adding a detention pond at the outlet

2. Purpose
Illustrate data preparation and entry to model storage devices in HEC-HMS
Uses Example 3B as basis.

3. Learning Objectives
Learn how to select canopy and depression storage abstractions.
Will use to examine effects
Learn how to prepare storage-discharge tables.
Use Excel and typical hydraulics considerations to build the table.
Learn how to import the storage-discharge table into HEC-HMS
Paired-Data manager

4. Problem Statement
Simulate the response of the Ash Creek watershed at Highland Road for 20 May 1978 historical conditions.
Use Example 3B as the base “model”
Use Example 3B output hydrograph as a comparison hydrograph to examine effect of different abstractions on simulation
Treat the entire watershed as a single sub-basin.

5. Background and Data Watershed Outlet
Highland Road and Ash Creek, Dallas, TX.
Area is residential subdivisions, light industrial parks, and some open parkland.
White Rock Lake is water body to the North-West

6. Physical Properties Watershed Properties AREA=6.92 mi2 MCL=5.416 mi
MCS=
CN=86
R=0

7. Loss Model: Green-Ampt
Parameter estimation
Initial water content
Saturated water content: 0.464
Saturated hydraulic conductivity: 0.04 in/hr
Soil suction: 8.27 inches

8. Preparing Example 4 “Clone” Example 3B and rename the project.
Select path
“Save-As”

9. Example 4 Run Example 4 Will use skills developed in Example 3
Export the simulation hydrograph to a file.
Re-import that file as a discharge gage.
We will use this hydrograph to examine effect of abstraction changes.
Will use skills developed in Example 3
External hydrograph import
Develop new skill – exporting results

10. Prepare a Base-Case Run Example 4
Export the simulation hydrograph to a file.

11. Export/Import Base Case
Run Example 4
Import the result hydrograph to a discharge gage.

12. Verify Import Re-Run Example 4
Set the result hydrograph as the “observation” discharge gage.
Observe “perfect” fit
Diagnostic in practical models, perfect fits mean analyst chose wrong time series.

13. Canopy Abstraction Set canopy abstraction to some non-zero value.
What do we expect discharge to do?

14. Canopy Abstraction Set canopy abstraction to some non-zero value.
What do we expect discharge to do?
Storing up to ½ inch of precipitation – anticipate that peak is reduced and a loss of early runoff.

15. Canopy Abstraction Set canopy abstraction to some non-zero value.
What do we expect discharge to do?
Storing up to ½ inch of precipitation – anticipate that peak is reduced and a loss of early runoff
Run the new model and examine results

16. Depression Abstraction
Similar procedure and data-entry items.
What do we expect discharge to do?
Storing even more precipitation, anticipate further reduction in peak.
Run the new model and examine results

17. Reservoir Storage
Now will return to more reasonable canopy and depression values
Canopy = 0.05 in
Depression = 0.25 in
Will use as a basis before adding a reservoir element

18. Reservoir Storage Reservoir element Locate on the basin
Use toolbars in HEC-HMS
Identify connection
Use connections in HEC-HMS
Specify a storage-discharge relation
Build table externally (in Excel) then import information into HEC-HMS

19. Example 4 Reservoir element Locate on the basin
Use toolbars in HEC-HMS

20. Reservoir Storage Connections in HEC-HMS
Hydrologic/Hydraulic Elements are “connected” by assigning to an element its downstream receiving element.
Rainfall thus connects “downstream” to a sub-basin.
In this example, the sub-basin will drain “downstream” to the reservoir
The reservoir will then drain “downstream” to our outlet of interest (i.e. its output is the outlet)

21. Reservoir Storage Reservoir element
Identify the reservoir as “downstream” of the sub-basin
In the sub-basin editor, we select the reservoir as the downstream element

22. Reservoir Storage Reservoir element
Identify the reservoir as “downstream” of the sub-basin
In the sub-basin editor, we select the reservoir as the downstream element
If successful, then the connection is shown on the GUI

23. Reservoir Storage Reservoir element
Flow from Sub-Basin
Reservoir element
Need to build the storage-discharge table
External to HMS
Analyze the reservoir hydraulics
This example, pretend an 8-foot deep detention basin fit into the outlet somehow

24. Reservoir Storage Reservoir element Analyze the reservoir hydraulics
discharge

25. Reservoir Storage
Storage-Discharge is entered in the “Paired Data “ structure
Components
Paired Data
New

26. Example 4 Storage-Discharge Table by cut-paste from the worksheet
Need storage in Acre-Feet

27. Reservoir Storage Run the model
Turned off the canopy and depression storage so can compare effect of reservoir.

28. Reservoir Storage
What if Reservoir is increased to 16X larger – about 0.5% of entire drainage area?
Notice the shifting in peak location and magnitude
Changed:
Multiplied the storage by 16.
Used the watershed outlet as the source for reference curve

29. HEC-HMS Example 4 Learning Points
Copy entire projects to keep different models organized.
Canopy and Depression Storage are abstractions – impact the excess precipitation.
Reservoir Storage attenuates the runoff signal, and if sufficient delays peak arrival.
Stream routing will look similar to reservoir behavior, parameterization is different.

30. HEC-HMS Example 4 Learning Points
Used external analysis to build the storage-discharge table
Simplified hydraulics to account for weir and orifice flow.
Excel to produce the STORAGE-DISCHARGE table.
Paired Data manager for HMS table
Imported the Excel table into the Storage-Discharge table in HMS

31. HEC-HMS Example 4 Learning Points
Used output time series to build comparison hydrographs to examine effect of changes
Use of a hand-written simulation log and some forethought could allow multiple comparisons in a single project directory – file count could get large, naming convention would be important.

32. HEC-HMS Example 4 Learn more Next module HEC HMS user manual
FHWA-NHI Highway Hydrology
Next module
Routing